LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices. [1]

LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.

The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.

When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.

LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.

LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.

LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. [2] Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.

LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs. [3]

LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states “less than 100ns.” LEDs used in communications devices can have even faster response times.

LEDs can be very small and are easily populated onto printed circuit boards.

LEDs do not contain mercury, unlike compact fluorescent lamps.

Due to the human eye's visual persistence LED's can be pulse width or duty cycle modulated in order to save power or achieve an apparent higher brightness for a given power input. The eye will tend to perceive the peak current light level rather than the average current light level when the modulation rate is higher than approximately 1000 hertz and the duty cycle is greater than 15 to 20%. This is also useful when applied to the multiplexing used in 7-segment displays.

Lighting LEDs on low voltage
LEDs are normally operated in parallel strings of series LEDs, with the total LED voltage typically adding up to around 2/3 of the supply voltage, and resistor current control for each string.
In resistor-drive devices, LED current is then proportional to power supply (PSU) voltage minus total LED string voltage. Where battery sources are used, the PSU voltage can vary widely, causing large changes in LED current and therefore color and light output. For such applications, a constant current regulator is preferred to resistor control. Low drop-out (LDO) constant current regs also allow the total LED string voltage to be a higher percentage of PSU voltage, resulting in improved efficiency and reduced power use.
Torches run one or more lighting LEDs on a low voltage battery. These usually use a resistor ballast.
In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type, such as one using a different battery chemistry.